1. Field of the Invention
The present invention relates to a minute droplet forming method and minute droplet forming apparatus applicable to various solutions.
2. Related Background Art
A method utilizing electrostatic attraction has conventionally been known as a method for forming a droplet. This method is one in which a pulse voltage is applied between a nozzle containing a liquid for forming a droplet and a substrate arranged to face a nozzle tip acting as a droplet dropping port, so as to attract the liquid from the nozzle tip toward the substrate by an electrostatic force, whereby thus formed droplet is caused to drop onto the substrate. According to this method, the formed droplet has larger and smaller sizes as the peak value of the applied pulse voltage is raised and lowered, respectively, whereby the size of the formed droplet can be controlled when the peak value is regulated.
In the above-mentioned droplet forming method based on the electrostatic attraction, however, the size of the formed droplet depends on the diameter of the nozzle tip, whereby droplets having a predetermined size or smaller cannot be formed. Namely, as the peak value of the pulse voltage applied for forming a minute droplet is lowered, the electrostatic force fails to overcome the surface tension occurring at the nozzle tip at a certain peak value or lower, thereby forming no droplets. Therefore, it is necessary to use a nozzle having a small tip diameter when forming a minute droplet. Nozzles having a small diameter, however, are problematic in that they are frequently clogged with dust and the like contained in the liquid.
Therefore, it is an object of the present invention to provide a minute droplet forming method and minute droplet forming apparatus solving the problem mentioned above.
For solving the above-mentioned problem, the minute droplet forming method in accordance with the present invention is a minute droplet forming method of electrostatic attraction type for forming a minute droplet by attracting a liquid by applying a pulse voltage to a nozzle tip containing the liquid, the method comprising a step of applying the pulse voltage between a substrate arranged to face the nozzle tip with a predetermined space therebetween and the liquid within the nozzle so as to project the liquid from the nozzle tip and form a liquid column, and a step of isolating the droplet by enhancing a fluid resistance within said nozzle so as to cause a setback force for returning said liquid into said nozzle to act on said formed liquid column.
The minute droplet forming apparatus in accordance with the present invention, on the other hand, comprises (1) a nozzle for storing therewithin a liquid for forming a droplet; (2) a substrate, arranged so as to face a tip of the nozzle, for mounting the droplet dropped from the nozzle tip; (3) a pulse power supply for applying a pulse voltage between the liquid within the nozzle and the substrate; (4) a fluid regulating unit adapted to change a fluid resistance within said nozzle; and (5) a control unit for controlling the pulse power supply and the fluid regulating unit.
In the minute droplet forming method and apparatus in accordance with the present invention, a liquid column, which is a liquid drawn out of the nozzle tip, is returned into the nozzle by the setback force, whereby a droplet is isolated from the liquid column. Thus isolating the droplet makes it possible to form a droplet having a diameter smaller than the nozzle diameter. For causing the setback force to act, in the present invention the fluid resistance within the nozzle is raised so as to slow down the velocity of flow generated within the nozzle by the electrostatic force, thus forming a negative pressure at the nozzle tip part, which is utilized as the setback force.
Thus controlling the setback force makes it possible to adjust the size of the formed droplet without changing the diameter of the nozzle.
It will be preferable if each of the forming and isolating of droplets is carried out under a saturation vapor pressure, since thus formed droplets become hard to evaporate.
Preferably, the nozzle is a core nozzle having a core arranged within the nozzle. When the nozzle is a core nozzle as such, the influence of surface tension can be lowered.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it is clear that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.